用Java编写一个词法分析器

好的，我可以为您提供一个基本的Java词法分析器代码，如下：

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Lexer {
    private String input;
    private int currentPosition;
    private Token currentToken;

    public Lexer(String input) {
        this.input = input;
        this.currentPosition = 0;
        this.currentToken = null;
    }

    private boolean isWhitespace(char c) {
        return c == ' ' || c == '\t' || c == '\n' || c == '\r';
    }

    private boolean isDigit(char c) {
        return c >= '0' && c <= '9';
    }

    private boolean isLetter(char c) {
        return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
    }

    private Token getNextToken() {
        if (currentPosition >= input.length()) {
            return null;
        }

        char currentChar = input.charAt(currentPosition);

        if (isWhitespace(currentChar)) {
            currentPosition++;
            return getNextToken();
        }

        if (isDigit(currentChar)) {
            StringBuilder value = new StringBuilder();

            while (isDigit(currentChar)) {
                value.append(currentChar);
                currentPosition++;

                if (currentPosition >= input.length()) {
                    break;
                }

                currentChar = input.charAt(currentPosition);
            }

            return new Token(TokenType.NUMBER, value.toString());
        }

        if (isLetter(currentChar)) {
            StringBuilder value = new StringBuilder();

            while (isLetter(currentChar) || isDigit(currentChar)) {
                value.append(currentChar);
                currentPosition++;

                if (currentPosition >= input.length()) {
                    break;
                }

                currentChar = input.charAt(currentPosition);
            }

            return new Token(TokenType.IDENTIFIER, value.toString());
        }

        switch (currentChar) {
            case '+':
                currentPosition++;
                return new Token(TokenType.PLUS, "+");
            case '-':
                currentPosition++;
                return new Token(TokenType.MINUS, "-");
            case '*':
                currentPosition++;
                return new Token(TokenType.MULTIPLY, "*");
            case '/':
                currentPosition++;
                return new Token(TokenType.DIVIDE, "/");
            case '(':
                currentPosition++;
                return new Token(TokenType.LEFT_PAREN, "(");
            case ')':
                currentPosition++;
                return new Token(TokenType.RIGHT_PAREN, ")");
            default:
                throw new RuntimeException("Invalid character: " + currentChar);
        }
    }

    public Token getCurrentToken() {
        if (currentToken == null) {
            currentToken = getNextToken();
        }

        return currentToken;
    }

    public Token getNextNonWhitespaceToken() {
        Token token = getNextToken();

        while (token != null && token.getType() == TokenType.WHITESPACE) {
            token = getNextToken();
        }

        currentToken = token;

        return token;
    }
}

enum TokenType {
    NUMBER,
    IDENTIFIER,
    PLUS,
    MINUS,
    MULTIPLY,
    DIVIDE,
    LEFT_PAREN,
    RIGHT_PAREN,
    WHITESPACE
}

class Token {
    private TokenType type;
    private String value;

    public Token(TokenType type, String value) {
        this.type = type;
        this.value = value;
    }

    public TokenType getType() {
        return type;
    }

    public String getValue() {
        return value;
    }

    @Override
    public String toString() {
        return "Token{" +
                "type=" + type +
                ", value='" + value + '\'' +
                '}';
    }
}

这是一个简单的词法分析器，它可以识别数字、标识符和一些基本的运算符。您可以根据需要进行修改和扩展。